1. Field of the Invention
The present invention relates to a magnetic memory device, and more particularly to a magnetic memory device using a magnetoresistive effect element as a memory cell.
2. Description of the Related Art
Various types of magnetic memories have been proposed so far. Further, a magnetic random access memory (MRAM) using magnetoresistive elements that exhibit a giant magnetoresistive effect (GMR) has recently been proposed. In particular, attention is now being paid to an MRAM using a ferromagnetic tunnel junction.
The ferromagnetic tunnel junction is formed of three layers, e.g., a first ferromagnetic layer, an insulation layer and a second ferromagnetic layer. In this structure, a current flows through the insulation layer by tunneling. The resistance of the junction varies in accordance with the cosine concerning the angle between the direction of magnetization of the first ferromagnetic layer and that of the second ferromagnetic layer. Accordingly, the junction resistance assumes the minimum value when the direction of magnetization of the first ferromagnetic layer is parallel with that of the second ferromagnetic layer, and assumes the maximum value when the former is anti-parallel with the latter. This is called a tunneling magnetoresistive (TMR) effect, and there is a case where the rate of change in the junction resistance due to the TMR exceeds 70% at room temperature.
In a memory cell including a ferromagnetic tunnel junction, at least one ferromagnetic layer is regarded as a base layer, and has its direction of magnetization fixed, and the other ferromagnetic layer is used as a recording layer. In this memory cell, data is recorded by making digital information (“0”, “1”) correspond to the parallel or anti-parallel state, concerning direction of magnetization, of the base layer and recording layer. Writing of data to the memory cell is realized by switching the direction of magnetization of the recording layer, using a magnetic field that is generated by a current guided to write wiring provided for the memory cell.
Further, reading of data from the memory cell is realized by guiding a current to the ferromagnetic tunnel junction and detecting a change in the resistance of the junction due to the TMR effect. A large number of memory cell similar to the above-described memory cell are arranged into a magnetic memory. More specifically, to select an arbitrary cell, a switching transistor is provided for each cell as in, for example, a dynamic random access memory (DRAM), and a peripheral circuit is incorporated. Further, a device, in which a ferromagnetic tunnel junction and diode are provided at the intersection of a word line and bit line, has been proposed (see U.S. Pat. Nos. 5,640,343 and 5,650,958).
To operate an MRAM using memory cells that include ferromagnetic tunnel junctions, it is always necessary to eliminate erroneous writing of data to non-selected cells. In MRAMs, a magnetic field acquired by synthesizing magnetic field Hx, generated in the direction of the axis of easy magnetization, with magnetic field Hy generated in the direction of the axis of hard magnetization is applied to each selected cell, thereby writing data thereto. At this time, no magnetic field is applied or only a one-directional magnetic field is applied to the non-selected cells. Note that the memory cell to which only a one-directional magnetic field is applied is called an incompletely selected cell.
The magnetization switching characteristic in a simultaneous rotation model is expressed by the asteroid curve. As can be understood from the asteroid curve, switching magnetic field intensity Hsw necessary for magnetization switching when a magnetic field is applied in both the direction of easy magnetization and the direction of hard magnetization is lower than easy-axis switching magnetic field intensity Hc. At this time, one-directional magnetic field intensity Hx necessary to write data to a selected memory cell can be set to a value lower than Hc. Accordingly, theoretically, erroneous writing of data to any incompletely selected memory cell can be prevented. Actually, however, variations in switching magnetic field intensity exist, which may cause data to be erroneously written to an incompletely selected memory cell unless Hsw is sufficiently lower than Hc.
On the other hand, since magnetic random access memories function as non-volatile memories, they are required to hold record data in a stable manner. There is a parameter called a thermal fluctuation constant and used as a target for reliably recording data for a long time. It is known that the thermal fluctuation constant is proportional to the volume of the recording layer and switching magnetic field intensity Hsw. Therefore, if switching magnetic field intensity Hsw is reduced to reduce the rate of erroneous writing, the thermal stability is also reduced, with the result that data cannot be held for a long time.
In light of the above, to put a highly integrated magnetic memory into practice, it is very important to propose a magnetoresistive effect element capable of holding data for a long time by enhancing the thermal stability with switching magnetic field intensity Hsw reduced.
Further, Japanese Patent No. 3548036 discloses a technique, related to the above, for correcting the magnetization pattern of a magnetoresistive element to enhance the write characteristics.